To obtain improved print quality in a thermal ink jet printer, a substantially uniform gap is desired between a nozzle plate surface having nozzles through which the ink is ejected as droplets and a sheet of media to which the droplets are applied by transverse motion of the nozzle plate relative to the sheet of media while the sheet of media is stationary. During advancement of the sheet of media relative to the nozzles in the nozzle plate, no printing occurs.
With relatively thin sheets of media such as bond paper having a thickness of about 0.1 mm, for example, buckling of the leading edge of the sheet of media by its engagement with an inclined surface maintains a substantially uniform gap between the sheet of media and the nozzle plate surface. That is, absorption of the selectively applied ink droplets by a buckling sheet of media does not create an uncontrolled buckling of the sheet of media to significantly vary the gap between the sheet of media and the nozzle plate surface.
Because of the relatively small gap, approximately 1 mm, a relatively thick sheet of media such as an envelope having a thickness of about 0.5 mm, for example, will buckle; the sheet will either engage the nozzle plate surface or be very close thereto so as to graze it. With the number of nozzles utilized and the speed of advancement of the sheet of media necessary for satisfactory throughput for a relatively thin sheet of media such as bond paper, for example, the relatively thick sheet of media will not have the ink dry thereon when it grazes or engages the nozzle plate surface adjacent where it is buckled. Accordingly, this grazing or engaging of the relatively thick sheet of media with the nozzle plate surface produces ink smear or smudge.
Therefore, the substantially uniform gap remains with printing on a relatively thin sheets of media. However, the gap does not remain substantially uniform with relatively thick sheets of media such as envelopes, for example.